The harsh environment and stringent reliability requirements of space technology require detailed knowledge of the motion of all mechanical devices. The failure modes must be well understood and catalogued. While this may be a straightforward analysis for macroscopic devices, microscopic mechanical structures require new tools to study their response under various conditions. In the existing art, static methods are generally employed and the motion of micro-electromechanical systems (MEMS) is then analytically derived. These inferential results are unacceptable for the requirements of modern aerospace applications and are often lacking in their precision when related to dynamic operation. Thorough qualification of micro-mechanical systems requires directly observed, reliable data, lacking in conventional metrology systems.
To study the motion of microscopic mechanical parts, it is not practical to load them with sensors. Non-contact measurement techniques are at a premium for dynamic MEMS device measurement. Optical methods of interrogation are ideal under these restrictions, and such schemes are well documented in the literature, offering high resolution and rapid measurement with a non-contact technique. Test products using laser-Doppler vibrometry or white light interferometry are commercially available on the market today. These products, however, have serious drawbacks. The vibrometer only yields velocity data, not absolute shape or displacement. In addition to this failing, the vibrometer measures single points at a time, requiring accurate scanning and stitching over the entire surface of the test object. The actual motion and shape of the device must then be gained inferentially through analysis and is subject to many sources of error, both in the scanning and interpretation of device measurements.
The white light interferometer has similar problems. Although it can take an image rather than a point of data at a time, the object or reference mirror must be scanned to find the interference maxima at each imaging pixel. While this does generate displacement data over a large dynamic range, the long scan time relies on very highly repeated motion. Couple this requirement to a slightly noisy environment and the data becomes riddled with potential errors.
A conventional state-of-the-art solution that addresses these issues is the stroboscopic Michelson Interferometer or, a subset of those, the Twyman-Green interferometer. The test system can achieve sub-nanometer resolution interferometricly and diffraction limited lateral resolution using microscopic objectives. Although the Twyman-Green interferometer relies on repeated motion of the device for dynamic characterization, only five to six vertical scan steps are done per time step of device motion. Several configurations have been documented in which the interferometer outputs to an imaging system. When implemented with a camera or video device, this records data from a broad area of the device so no lateral scanning is required. The conventional Michelson or Twyman-Green test system is composed of bulk optical components. These tend to include a laser light source to illuminate the device and reference mirror, an optical beam splitter, and optical microscope objectives to image the sample onto a camera. The standard configuration is such that the illuminating optical beam is split at the beam splitter. One of the two outputs of the beam splitter proceeds to the reference mirror of the interferometer and is reflected back to the beam splitter. The second output beam is directed to the device under test, which also reflects it back to the beam splitter. These beams then recombine at the beam splitter and are directed to the camera to be recorded. When the test and reference arms have a path length within the coherence length of the source, interference fringes will cover the image. The choice of optical source is non-trivial. When the source has a very high degree of coherence, the tolerances on distance are eased, but the final image will be spotted with speckle patterns and interference from stray reflections, apertures and dust. When the coherence is very low, for example in a white light interferometer, the image will be crisp but interference will only occur over very narrow displacements. To avoid constant scanning of the reference mirror, the coherence length must be greater than twice the characteristic dimensions of the device under test.
To perform measurements on micrometer scale devices and samples, the conventional Michelson test system employs a microscope objective between the beam splitter and the test sample. This system relies on the interference of nearly identical optical fields so the optical paths to the test sample and reference mirror must be closely matched. This requires a second, identical microscope objective in the reference path so that the phase fronts have identical curvature. The second objective puts even further constraints on the allowed displacement tolerances. When the reference mirror is further from the objective than the test device, the phase front will have a different curvature and the fringes will show a warped surface. This creates a very difficult problem in calibrating the system. If one were to calibrate the system by inserting an optically flat mirror in place of the test sample, one would have to assure that the test sample was the same axial distance from the microscope when testing. This exact length tolerance would depend on the details. of the system but can be expected to be on the order of tens of micrometers. Achieving such precision is not practical in a manufacturing or testing environment.
In a conventional interferometer, vibration in any part of either optical path, that is, in the microscope objective path, reference mirror path, or at the device, is translated into measurement errors. This presents a serious obstacle to high fidelity metrology. Because interferometric systems are designed to measure distances on the order of nanometers, they are also sensitive to vibrations on that length scale as well. Currently, these types of microscopic interferometers disadvantageously require floating optical tables and controlled laboratory environments. These and other disadvantages are solved or greatly reduced using the present invention.